The importance of achieving accurate mutual alignment of individual components in any optical system is well known. The miniature dimensions of components used in modern optical communication systems render such alignment very difficult both to achieve and to maintain. For example, one of the main problems in the construction of semiconductor laser transmitters is that of efficiently coupling the optical power from a semiconductor laser diode into a monomode fibre. To obtain efficient coupling, the fibre end must be very precisely aligned with the emitting area of the laser facet. When such alignment is achieved, the fibre must then be fixed in place, ideally by a method which ensures that alignment is sustained throughout the device lifetime.
In the past various methods of mounting have been proposed or employed in an effort to meet the required demands of accuracy and longevity of alignment. Traditionally, components have been fixed by soldering. However, where a number of separate components are to be mounted on the same substrate a series of solders must usually be used, with each successive component being fixed with a progressively lower melting point solder. The first mounted components are not then loosened when others are fixed subsequently. Nevertheless, there are obvious problems if it is necessary, for maintainance purposes, for example, to remove a first mounted component, fixed with a high melting point solder, without disturbing those later mounted components, fixed with lower melting point solders.
Some methods of restricting the applied soldering heat to the immediate locality of a component have been tried in an attempt to avoid the problems involved in the use of a series of solders. Such methods include the use of a hot gas jet, or direct thermal conduction from a heated clamp. However, because of drawbacks including difficulties in ensuring adequate heat transfer and, more significantly, the difficulty of avoiding alignment perturbations, these techniques have generally proved unsatisfactory, even for laboratory applications.
The adoption of organic adhesives has been successful for fixing some components, but such adhesives are not generally acceptable for several reasons. Firstly, component replacement is considered difficult, if not wholly impractical. Secondly, during aging, adhesives may be subject to shrinkage, with the likelihood of adverse consequences on component alignment. Thirdly, also with ageing, adhesives are liable to outgas undesirable organic substances. Enclosed in component packages, such as, for example, a laser transmitter, these substances can cause severe degradation and premature failure of the components.
It is one object of the present invention to provide a method of mounting optical components, which permits some of the above disadvantages to be overcome or mitigated.
It is a second object of the present invention to provide an improved optical component adapted to facilitate mounting by the improved method.
According to the present invention there is provided a method of mounting an optical component on a substrate, the method comprising providing a component with dedicated heating means and fixing the component to the substrate with solder, wherein the component and solder are heated locally by the dedicated heating means.
Preferably the temperature of the heating means is electrically controlled.
Conveniently, the heating means comprises a resistive heating element.
The heating means may be provided by bonding or otherwise affixing a miniature resistor to the component.
Alternatively, the heating means may be provided by deposition of a resistive ink film on a suitably insulated surface on the component. The insulated surface may be a ceramic layer. Metallised contacts may be provided to allow simple electrical connection to the resistive film.
According to a second aspect of the present invention, an optical component comprises dedicated heating means to facilitate solder mounting of the component.
The heating means may be a resistive ink film deposited on an insulating surface on the component, and the resistive film may have metallised contacts for electrical connections.